RFC 3404 DDDS Based URI Resolution October 2002
Separating the way names are resolved from the way they are
constructed provides several benefits. It allows multiple naming
approaches and resolution approaches to compete, as it allows
different protocols and resolvers to be used. There is just one
problem with such a separation - how do we resolve a name when it
can't give us directions to its resolver?
For the short term, the Domain Name System (DNS) is the obvious
candidate for the resolution framework, since it is widely deployed
and understood. However, it is not appropriate to use DNS to
maintain information on a per-resource basis. First of all, DNS was
never intended to handle that many records. Second, the limited
record size is inappropriate for catalog information. Third, domain
names are not appropriate as URNs.
Therefore our approach is to use the DDDS to locate "resolvers" that
can provide information on individual resources, potentially
including the resource itself. To accomplish this, we "rewrite" the
URI into a Key following the rules found in the DDDS. This document
describes URI Resolution as an application of the DDDS and specifies
the use of at least one Database based on DNS.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
All capitalized terms are taken from the vocabulary found in the DDDS
algorithm specification found in RFC 3403 [3].
3. The Distinction Between URNs and URIs
From the point of view of this system, there is no theoretical
difference between resolving URIs in the general case and URNs in the
specific case. Operationally however, there is a difference that
stems from URI resolution possibly not becoming of widespread use.
If URN resolution is collapsed into generic URI resolution, URNs may
suffer by the lack of adoption of URI resolution.
The solution is to allow for shortcutting for URN resolution. In the
following specification generic URI resolution starts by inserting
rules for known URI schemes into the 'uri.arpa.' registry. For the
'URN:' URI scheme, one of the rules found in 'uri.arpa.' would be for
the 'urn' URI scheme. This rule would simply delegate to the
'urn.arpa.' zone for additional NAPTRs based on the URN namespace.
Essentially, the URI Resolution Rewrite Rule for 'URN:' is the URN
Resolution Application's First Well Known Rule.
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RFC 3404 DDDS Based URI Resolution October 2002
Therefore, this document specifies two DDDS Applications. One is for
URI Resolution and the other is for URN Resolution. Both are
technically identical but by separating the two URN Resolution can
still proceed without the dependency.
4. The URI and URN Resolution Application Specifications
This template defines the URI and URN Resolution DDDS Application
according to the rules and requirements found in [3]. The DDDS
database used by this Application is found in [4] which is the
document that defines the Naming Authority Pointer (NAPTR) DNS
Resource Record (RR) type.
4.1 Application Unique String
The Application Unique String is the URI or URN for which an
authoritative server is being located. This URI or URN MUST be
canonicalized and hex encoded according to the "absolute-uri"
production found in the Collected ABNF from RFC 2396 [15].
4.2 First Well Known Rule
In the URI case, the first known key is created by taking the URI
scheme. In the URN case, the first known key is the Namespace
Identifier. For example, the URI 'http://www.example.com/' would
have a 'http' as its Key. The URN 'urn:foo:foospace' would have
'foo' as its first Key.
4.3 Flags
At this time only four flags, "S", "A", "U", and "P", are defined.
The "S", "A" and "U" flags are for a terminal lookup. This means
that the Rule is the last one and that the flag determines what the
next stage should be. The "S" flag means that the output of this
Rule is a domain-name for which one or more SRV [9] records exist.
See Section 5 for additional information on how URI and URN
Resolution use the SRV record type. "A" means that the output of the
Rule is a domain-name and should be used to lookup either A, AAAA, or
A6 records for that domain. The "U" flag means that the output of
the Rule is a URI [15].
The "P" flag says that the remainder of the DDDS Algorithm is ignored
and that the rest of the process is application specific and outside
the scope of this document. An application can use the Protocol part
found in the Services field to identify which Application specific
set of rules that should be followed next. The record that contains
the 'P' flag is the last record that is interpreted by the rules in
this document. One might think that this would also make the "P"
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RFC 3404 DDDS Based URI Resolution October 2002
flag an indicator of a terminal lookup but this would be incorrect
since a "terminal" Rule is a DDDS concept and this flag indicates
that anything after this rule does not adhere to DDDS concepts at
all.
The remaining alphabetic flags are reserved for future versions of
this specification. The numeric flags may be used for local
experimentation. The S, A, U and P flags are all mutually exclusive,
and resolution libraries MAY signal an error if more than one is
given. (Experimental code and code for assisting in the creation of
Rewrite Rules would be more likely to signal such an error than a
client such as a browser.) It is anticipated that multiple flags
will be allowed in the future, so implementers MUST NOT assume that
the flags field can only contain 0 or 1 characters. Finally, if a
client encounters a record with an unknown flag, it MUST ignore it
and move to the next Rule. This test takes precedence over any
ordering since flags can control the interpretation placed on fields.
A novel flag might change the interpretation of the regexp and/or
replacement fields such that it is impossible to determine if a
record matched a given target.
The "S", "A", and "U" flags are called 'terminal' flags since they
halt the looping DDDS algorithm. If those flags are not present,
clients may assume that another Rule exists at the Key produced by
the current Rewrite Rule.
4.4 Services Parameters
Service Parameters for this Application take the form of a string of
characters that follow this ABNF:
service_field = [ [protocol] *("+" rs)]
protocol = ALPHA *31ALPHANUM
rs = ALPHA *31ALPHANUM
; The protocol and rs fields are limited to 32
; characters and must start with an alphabetic.
In other words, an optional protocol specification followed by 0 or
more resolution services. Each resolution service is indicated by an
initial '+' character.
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RFC 3404 DDDS Based URI Resolution October 2002
The empty string is also valid. This will typically be seen at the
beginning of a series of Rules, when it is impossible to know what
services and protocols will be offered at the end of a particular
delegation path.
4.4.1 Services
The service identifiers that make up the 'rs' production are generic
for both URI and URN resolution since the input value types itself
based on the URI scheme. The list of valid services are defined in
[11].
Examples of some of these services are:
I2L: given a URI return one URI that identifies a location where the
original URI can be found.
I2Ls: given a URI return one or more URIs that identify multiple
locations where the original URI can be found.
I2R: given a URI return one instance of the resource identified by
that URI.
I2Rs: given a URI return one or more instances of the resources
identified by that URI.
I2C: given a URI return one instance of a description of that
resource.
I2N: given a URI return one URN that names the resource (Caution:
equality with respect to URNs is non-trivial. See [6] for
examples of why.)
4.4.2 Protocols
The protocol identifiers that are valid for the 'protocol' production
MUST be defined by documents that are specific to URI resolution. At
present the THTTP [10] protocol is the only such specification.
It is extremely important to realize that simply specifying any
protocol in the services field is insufficient since there are
additional semantics surrounding URI resolution that are not defined
within the protocols. For example, if Z39.50 were to be specified as
a valid protocol it would have to additionally define how it would
encode requests for specific services, how the URI is encoded, and
what information is returned.
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RFC 3404 DDDS Based URI Resolution October 20024.4.3 Applicability of Services
Since it is possible for there to be a complex set of possible
protocols and services a client application may often need to apply a
more complex decision making process to a set of records than simply
matching on an ordered list of protocols. For example, if there are
4 rules that are applicable the last one may have a more desirable
Service field than the first. But since the client may be satisfied
by the first it will never know about the 4th one which may be
'better'.
To mitigate this the client may want to slightly modify the DDDS
algorithm (for this application only!) in order to determine if more
applicable protocols/services exist. This can safely be done for
this application by using a more complex interaction between steps 3
and 4 of the DDDS algorithm in order to find the optimal path to
follow. For example, once a client has found a rule who's
Substitution Expression produces a result and who's Service
description is acceptable, it may make note of this but continue to
look at further rules that apply (all the while adhering to the
Order!) in order to find a better one. If none are found it can use
the one it made note of.
Keep in mind that in order for this to remain safe, the input to step
3 and the output of step 4 MUST be identical to the basic algorithm.
The client software MUST NOT attempt to do this optimization outside
a specific set of Rewrite Rules (i.e., across delegation paths).
4.5 Valid Databases
At present only one DDDS Database is specified for this Application.
"Dynamic Delegation Discovery System (DDDS) Part Three: The Domain
Name System (DNS) Database" (RFC 3403) [4] specifies a DDDS Database
that uses the NAPTR DNS resource record to contain the rewrite rules.
The Keys for this database are encoded as domain-names.
The output of the First Well Known Rule for the URI Resolution
Application is the URI's scheme. In order to convert this to a
unique key in this Database the string '.uri.arpa.' is appended to
the end. This domain-name is used to request NAPTR records which
produces new keys in the form of domain-names.
The output of the First Well Known Rule of the URN Resolution
Application is the URN's namespace id. In order to convert this to a
unique key in this Database the string '.urn.arpa.' is appended to
the end. This domain-name is used to request NAPTR records which
produces new keys in the form of domain-names.
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RFC 3404 DDDS Based URI Resolution October 2002
DNS servers MAY interpret Flag values and use that information to
include appropriate SRV and A records in the Additional Information
portion of the DNS packet. Clients are encouraged to check for
additional information but are not required to do so. See the
Additional Information Processing section of RFC 3404 for more
information on NAPTR records and the Additional Information section
of a DNS response packet.
The character set used to encode the substitution expression is
UTF-8. The allowed input characters are all those characters that
are allowed anywhere in a URI. The characters allowed to be in a Key
are those that are currently defined for DNS domain-names. The "i"
flag to the substitution expression is used to denote that, where
appropriate for the code points in question, any matches should be
done in a case-insensitive way.
5. Examples5.1 An Example Using a URN
Consider a URN that uses the hypothetical FOO namespace. FOO numbers
are identifiers for approximately 30 million registered businesses
around the world, assigned and maintained by Fred, Otto and Orvil,
Inc. The URN might look like:
urn:foo:002372413:annual-report-1997
The first step in the resolution process is to find out about the FOO
namespace. The namespace identifier [8], "foo", is extracted from
the URN and prepended to '.urn.arpa.', producing 'foo.urn.arpa.'.
The DNS is queried for NAPTR records for this domain which produces
the following results:
foo.urn.arpa.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "s" "foolink+I2L+I2C" "" foolink.udp.example.com.
IN NAPTR 100 20 "s" "rcds+I2C" "" rcds.udp.example.com.
IN NAPTR 100 30 "s" "thttp+I2L+I2C+I2R" "" thttp.tcp.example.com.
The order field contains equal values, indicating that no order has
to be followed. The preference field indicates that the provider
would like clients to use the special 'foolink' protocol, followed by
the RCDS protocol, and that THTTP is offered as a last resort. All
the records specify the "s" flag which means that the record is
terminal and that the next step is to retrieve an SRV record from DNS
for the given domain-name.
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RFC 3404 DDDS Based URI Resolution October 2002
The service fields say that if we speak of foolink, we will be able
to issue either the I2L, I2C or I2R requests to obtain a URI or ask
some complicated questions about the resource. The Resource
Cataloging and Distribution Service (RCDS) [12] could be used to get
some metadata for the resource, while THTTP could be used to get a
URI for the current location of the resource.
Assuming our client does not know the foolink protocol but does know
the RCDS protocol, our next action is to lookup SRV RRs for
rcds.udp.example.com, which will tell us hosts that can provide the
necessary resolution service. That lookup might return:
;; Pref Weight Port Target
rcds.udp.example.com IN SRV 0 0 1000 deffoo.example.com.
IN SRV 0 0 1000 dbexample.com.au.
IN SRV 0 0 1000 ukexample.com.uk.
telling us three hosts that could actually do the resolution, and
giving us the port we should use to talk to their RCDS server. (The
reader is referred to the SRV specification [9] for the
interpretation of the fields above.)
There is opportunity for significant optimization here. RFC 3404
defines that Additional Information section may be available. In
this case the the SRV records may be returned as additional
information for terminal NAPTRs lookups (as well as the A records for
those SRVs). This is a significant optimization. In conjunction
with a long TTL for *.urn.arpa. records, the average number of probes
to DNS for resolving most URIs would approach one.
Note that the example NAPTR records above are intended to represent
the result of a NAPTR lookup using some client software like
nslookup; zone administrators should consult the documentation
accompanying their domain name servers to verify the precise syntax
they should use for zone files.
Also note that there could have been an additional first step where
the URN was resolved as a generic URI by looking up urn.uri.arpa.
The resulting rule would have specified that the NID be extracted
from the URN and '.urn.arpa.' appended to it resulting in the new key
'foo.urn.arpa.' which is the first step from above.
5.2 CID URI Scheme Example
Consider a URI scheme based on MIME Content-Ids. The URI might look
like this:
cid:199606121851.1@bar.example.com
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RFC 3404 DDDS Based URI Resolution October 2002
(Note that this example is chosen for pedagogical purposes, and does
not conform to the CID URI scheme.)
The first step in the resolution process is to find out about the CID
scheme. The scheme is extracted from the URI, prepended to
'.uri.arpa.', and the NAPTR for 'cid.uri.arpa.' looked up in the DNS.
It might return records of the form:
cid.uri.arpa.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "" "!^cid:.+@([^\.]+\.)(.*)$!\2!i" .
Since there is only one record, ordering the responses is not a
problem. The replacement field is empty, so the pattern provided in
the regexp field is used. We apply that regexp to the entire URI to
see if it matches, which it does. The \2 part of the substitution
expression returns the string "example.com". Since the flags field
is empty, the lookup is not terminal and our next probe to DNS is for
more NAPTR records where the new domain is 'example.com'.
Note that the rule does not extract the full domain name from the
CID, instead it assumes the CID comes from a host and extracts its
domain. While all hosts, such as 'bar', could have their very own
NAPTR, maintaining those records for all the machines at a site could
be an intolerable burden. Wildcards are not appropriate here since
they only return results when there is no exactly matching names
already in the system.
The record returned from the query on "example.com" might look like:
example.com.
;; order pref flags service regexp replacement
IN NAPTR 100 50 "s" "z3950+I2L+I2C" "" z3950.tcp.example.com.
IN NAPTR 100 50 "s" "rescap+I2C" "" rescap.udp.example.com.
IN NAPTR 100 50 "s" "thttp+I2L+I2C+I2R" "" thttp.tcp.example.com.
Continuing with the example, note that the values of the order fields
are equal for all records, so the client is free to pick any record.
The Application defines the flag 's' to mean a terminal lookup and
that the output of the rewrite will be a domain-name for which an SRV
record should be queried. Once the client has done that, it has the
following information: the host, port, the protocol, and the services
available via that protocol. Given these bits of information the
client has enough to be able to contact that server and ask it
questions about the cid URI.
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RFC 3404 DDDS Based URI Resolution October 2002
Recall that the regular expression used \2 to extract a domain name
from the CID, and \. for matching the literal '.' characters
separating the domain name components. Since '\' is the escape
character, literal occurrences of a backslash must be escaped by
another backslash. For the case of the cid.uri.arpa record above,
the regular expression entered into the master file should be
"!^cid:.+@([^\\.]+\\.)(.*)$!\\2!i". When the client code actually
receives the record, the pattern will have been converted to
"!^cid:.+@([^\.]+\.)(.*)$!\2!i".
5.3 Resolving an HTTP URI Scheme
Even if URN systems were in place now, there would still be a
tremendous number of host based URIs. It should be possible to
develop a URI resolution system that can also provide location
independence for those URIs.
Assume we have the URI for a very popular piece of software that the
publisher wishes to mirror at multiple sites around the world:
http://www.example.com/software/latest-beta.exe
We extract the prefix, "http", and lookup NAPTR records for
'http.uri.arpa.'. This might return a record of the form:
http.uri.arpa. IN NAPTR
;; order pref flags service regexp replacement
100 90 "" "" "!^http://([^/:]+)!1!i" .
This expression returns everything after the first double slash and
before the next slash or colon. (We use the '!' character to delimit
the parts of the substitution expression. Otherwise we would have to
use backslashes to escape the forward slashes, and would have a
regexp in the zone file that looked like this:
"/^http:\\/\\/([^\\/:]+)/\\1/i").
Applying this pattern to the URI extracts "www.example.com". Looking
up NAPTR records for that might return:
www.example.com.
;; order pref flags service regexp replacement
IN NAPTR 100 100 "s" "thttp+L2R" "" thttp.example.com.
IN NAPTR 100 100 "s" "ftp+L2R" "" ftp.example.com.
Looking up SRV records for thttp.example.com would return information
on the hosts that example.com has designated to be its mirror sites.
The client can then pick one for the user.
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RFC 3404 DDDS Based URI Resolution October 20026. Notes
o Registration procedures for the 'urn.arpa.' and 'uri.arpa.' DNS
zones are specified in "Dynamic Delegation Discovery System (DDDS)
Part Five: URI.ARPA Assignment Procedures" (RFC 3405 [5].
o If a record at a particular order matches the URI, but the client
doesn't know the specified protocol and service, the client SHOULD
continue to examine records that have the same order. The client
MUST NOT consider records with a higher value of order. This is
necessary to make delegation of portions of the namespace work.
The order field is what lets site administrators say "all requests
for URIs matching pattern x go to server 1, all others go to
server 2".
o Note that SRV RRs impose additional requirements on clients.
7. IANA Considerations
The use of the "urn.arpa." and "uri.arpa." zones requires
registration policies and procedures to be followed and for the
operation of those DNS zones to be maintained. These policies and
procedures are spelled out in a "Dynamic Delegation Discovery System
(DDDS) Part Five: URI.ARPA Assignment Procedures (RFC 3405)" [5].
The operation of those zones imposes operational and administrative
responsibilities on the IANA.
The registration method used for values in the Services and Flags
fields is for a specification to be approved by the IESG and
published as either an Informational or standards track RFC.
The registration policies for URIs is found in RFC 2717 [17]. URN
NID registration policies are found in RFC 2611 [16].
8. Security Considerations
The use of "urn.arpa." and "uri.arpa." as the registry for namespaces
is subject to denial of service attacks, as well as other DNS
spoofing attacks. The interactions with DNSSEC are currently being
studied. It is expected that NAPTR records will be signed with SIG
records once the DNSSEC work is deployed.
The rewrite rules make identifiers from other namespaces subject to
the same attacks as normal domain names. Since they have not been
easily resolvable before, this may or may not be considered a
problem.
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RFC 3404 DDDS Based URI Resolution October 2002
Regular expressions should be checked for sanity, not blindly passed
to something like PERL.
This document has discussed a way of locating a resolver, but has not
discussed any detail of how the communication with the resolver takes
place. There are significant security considerations attached to the
communication with a resolver. Those considerations are outside the
scope of this document, and must be addressed by the specifications
for particular resolver communication protocols.
9. Acknowledgments
The editors would like to thank Keith Moore for all his consultations
during the development of this document. We would also like to thank
Paul Vixie for his assistance in debugging our implementation, and
his answers on our questions. Finally, we would like to acknowledge
our enormous intellectual debt to the participants in the Knoxville
series of meetings, as well as to the participants in the URI and URN
working groups.
Specific recognition is given to Ron Daniel who was co-author on the
original versions of these documents. His early implementations and
clarity of thinking was invaluable in clearing up many of the
potential boundary cases.
References
[1] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
[2] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Two: The Algorithm", RFC 3402, October 2002.
[3] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403, October
2002.
[4] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The Uniform Resource Identifiers (URI) Resolution
Application", RFC 3404, October 2002.
[5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Five: URI.ARPA Assignment Procedures", RFC 3405y, October 2002.
[6] Sollins, K. and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.
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RFC 3404 DDDS Based URI Resolution October 2002Appendix A. Pseudo Code
For the edification of implementers, pseudocode for a client routine
using NAPTRs is given below. This code is provided merely as a
convenience, it does not have any weight as a standard way to process
NAPTR records. Also, as is the case with pseudocode, it has never
been executed and may contain logical errors. You have been warned.
//
// findResolver(URN)
// Given a URN, find a host that can resolve it.
//
findResolver(string URN) {
// prepend prefix to ".urn.arpa."
sprintf(key, "%s.urn.arpa.", extractNS(URN));
do {
rewrite_flag = false;
terminal = false;
if (key has been seen) {
quit with a loop detected error
}
add key to list of "seens"
records = lookup(type=NAPTR, key); // get all NAPTR RRs for 'key'
discard any records with an unknown value in the "flags" field.
sort NAPTR records by "order" field and "preference" field
(with "order" being more significant than "preference").
n_naptrs = number of NAPTR records in response.
curr_order = records[0].order;
max_order = records[n_naptrs-1].order;
// Process current batch of NAPTRs according to "order" field.
for (j=0; j < n_naptrs && records[j].order <= max_order; j++) {
if (unknown_flag) // skip this record and go to next one
continue;
newkey = rewrite(URN, naptr[j].replacement, naptr[j].regexp);
if (!newkey) // Skip to next record if the rewrite didn't
match continue;
// We did do a rewrite, shrink max_order to current value
// so that delegation works properly
max_order = naptr[j].order;
// Will we know what to do with the protocol and services
// specified in the NAPTR? If not, try next record.
if(!isKnownProto(naptr[j].services)) {
continue;
}
if(!isKnownService(naptr[j].services)) {
continue;
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RFC 3404 DDDS Based URI Resolution October 2002
}
// At this point we have a successful rewrite and we will
// know how to speak the protocol and request a known
// resolution service. Before we do the next lookup, check
// the flags to see if we're done.
// Note: it is possible to rewrite this so that this valid
// record could be noted as such but continue on in order
// to find a 'better' record. But that code would be to
// voluminous and application specific to be illustrative.
if (strcasecmp(flags, "S")
|| strcasecmp(flags, "P"))
|| strcasecmp(flags, "A")) {
terminal = true;
services = naptr[j].services;
addnl = any SRV and/or A records returned as additional
info for naptr[j].
}
key = newkey;
rewriteflag = true;
break;
}
} while (rewriteflag && !terminal);
// Did we not find our way to a resolver?
if (!rewrite_flag) {
report an error
return NULL;
}
// Leave rest to another protocol?
if (strcasecmp(flags, "P")) {
return key as host to talk to;
}
// If not, keep plugging
if (!addnl) { // No SRVs came in as additional info, look them up
srvs = lookup(type=SRV, key);
}
sort SRV records by preference, weight, ...
for each (SRV record) { // in order of preference
try contacting srv[j].target using the protocol and one of the
resolution service requests from the "services" field of the
last NAPTR record.
if (successful)
return (target, protocol, service);
// Actually we would probably return a result, but this
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RFC 3404 DDDS Based URI Resolution October 2002
Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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